Bulhan, who is in custody, appeared at the Old Bailey before Mr Justice Spencer this morning where he pleaded not guilty to the charge of murder but guilty to manslaughter on the grounds of diminished responsibility.

He pleaded not guilty to charges of attempted murder, but pleaded guilty to charges of wounding with intent.

The cop said he and another officer had gone to Bedford Place to cut off Zakaria Bulhan's escape route after his stabbing spree.

The pair got out of the police car and chased after Bulhan after he refused to stop and drop his large kitchen knife.

He described the “surreal” scene as Bulhan pelted away from them with the knife held aloft as if he was about to strike again, glancing over his shoulder as they screamed at him to stop.

The 19-year-old suspect managed to dodge a colleague who tried to take him down using his baton and was moments away from diving around a corner into unsuspecting members of the public.

As he sprinted towards the British Museum, the officer said they made a "pre-emptive strike" with a Taser before he could hurt anyone else.

He said: "He was running at pace with the knife raised and kept looking back at us every time we were screaming to stop.

"He was nearing the corner with Great Russell Street. We knew already there were members of the public there.

"He may have been a danger, with unrestricted access to members of the public if we did not engage with him.

"At that point one of the others managed to withdraw his Taser and used that option. The Taser worked extremely effectively.

"He landed face first onto the floor and dropped the knife. We jumped on him, placed him in handcuffs and detained him."

Asked how near he came to shooting Bulhan dead, the officer said: "Very close. He was approaching that corner and we knew if he got around that corner for a very small amount of time he would have had unrestricted access to members of the public.

"It had to be a pre-emptive strike. We could not let him get around the corner."

Widower Richard Wagner said in a statement read in court: “Knowing what we now know, if we ever allow him to repeat his crime and he does so, the deficiencies will be our own and the blood will be on our hands.”

He and Ms Horton had been 'high school sweethearts' and Mr Wagner said: "Losing my wife and best friend under the circumstances has been as bad as you might imagine.

"As bad as I feel for my life, I feel worse for the loss experienced by my two daughters."

Prosecutor Mark Heywood QC said the pleas were accepted by the Crown having been considered "at the very highest level".

He told the judge: "At the time of these events it has been clearly established the defendant was suffering from an acute episode of a mental illness that has been diagnosed as paranoid schizophrenia.

"That acute episode was a psychotic one, and floridly so, at the time of these events on 3 August 2016."

The judge has said he will pass sentence tomorrow.

PROFILE OF A KILLER

The Russell Square killer did not receive the treatment he needed in the run up to his knife rampage, a court heard.

Bulhan's parents emigrated from Somalia to Norway in 1994 and he was born in the Scandinavian country three years later.

The second of three children, he moved to the UK with his mother in 2003.

Bulhan lived with his mother, step-father and siblings in Tooting.

He left school aged 16 and retook his GCSEs at college before dropping out in April 2016 as his mental state deteriorated.

He was referred to his local mental health trust in March 2016 and assed on 20 April, but received no further treatment despite acting increasingly 'oddly' in the period leading up to the attack.

The prosecutor said: "That is no doubt partly behind what follows.

"He was not then receiving the treatment for his condition to the extent that, as it now appears, was necessary, although not apparent at the time."

Bulhan's mother was visiting family in Holland in August and he had been to the East London Mosque, in Whitechapel on the evening of Wednesday 3 August.

The court heard he ran from the mosque and CCTV captured his movements later entering Russell Square from the south at 10.22pm.

He was seen moving slowly around the square before walking with purpose along the west footway moments before the attacks occurred. But the court heard none of the stabbings were caught on CCTV.

We pay for your stories! Do you have a story for The Sun Online news team? Email us at tips@the-sun.co.uk or call 0207 782 4368